1. Field of the Invention
This invention relates generally to a variable optical attenuator (VOA), and more particularly to an all-fiber acousto-optic tunable intensity attenuator that includes an acoustic wave device that reduces backward-propagating acoustic waves.
2. Description of Related Art
In modern telecommunication systems, many operations with digital signals are performed on an optical layer. For example, digital signals are optically amplified, multiplexed and demultiplexed. In long fiber transmission lines, the amplification function is performed by Erbium Doped Fiber Amplifiers (EDFA""s). The amplifier is able to compensate for power loss related to signal absorption, but it is unable to correct the signal distortion caused by linear dispersion, 4-wave mixing, polarization distortion and other propagation effects, and to get rid of noise accumulation along the transmission line. For these reasons, after the cascade of multiple amplifiers the optical signal has to be regenerated every few hundred kilometers. In practice, the regeneration is performed with electronic repeaters using optical-to-electronic conversion. However to decrease system cost and improve its reliability it is desirable to develop a system and a method of regeneration, or signal refreshing, without optical to electronic conversion. An optical repeater that amplifies and reshapes an input pulse without converting the pulse into the electrical domain is disclosed, for example, in the U.S. Pat. No. 4,971,417, Radiation-Hardened Optical Repeaterxe2x80x9d. The repeater comprises an optical gain device and an optical thresholding material producing the output signal when the intensity of the signal exceeds a threshold. The optical thresholding material such as polydiacetylene thereby performs a pulse shaping function. The nonlinear parameters of polydiacetylene are still under investigation, and its ability to function in an optically thresholding device has to be confirmed.
Another function vital to the telecommunication systems currently performed electronically is signal switching. The switching function is next to be performed on the optical level, especially in the Wavelength Division Multiplexing (WDM) systems. There are two types of optical switches currently under consideration. First, there are wavelength insensitive fiber-to-fiber switches. These switches (mechanical, thermo and electro-optical etc.) are dedicated to redirect the traffic from one optical fiber to another, and will be primarily used for network restoration and reconfiguration. For these purposes, the switching time of about 1 msec (typical for most of these switches) is adequate; however the existing switches do not satisfy the requirements for low cost, reliability and low insertion loss. Second, there are wavelength sensitive switches for WDM systems. In dense WDM systems having a small channel separation, the optical switching is seen as a wavelength sensitive procedure. A small fraction of the traffic carried by specific wavelength should be dropped and added at the intermediate communication node, with the rest of the traffic redirected to different fibers without optical to electronic conversion. This functionality promises significant cost saving in the future networks. Existing wavelength sensitive optical switches are usually bulky, power-consuming and introduce significant loss related to fiber-to-chip mode conversion. Mechanical switches interrupt the traffic stream during the switching time. Acousto-optic tunable filters, made in bulk optic or integrated optic forms, (AOTFs) where the WDM channels are split off by coherent interaction of the acoustic and optical fields though fast, less than about 1 microsecond, are polarization and temperature dependent. Furthermore, the best AOTF consumes several watts of RF power, has spectral resolution about 3 nm between the adjacent channels (which is not adequate for current WDM requirements), and introduces over 5 dB loss because of fiber-to-chip mode conversions.
Another wavelength-sensitive optical switch may be implemented with a tunable Fabry Perot filter (TFPF). When the filter is aligned to a specific wavelength, it is transparent to the incoming optical power. Though the filter mirrors are almost 100% reflective no power is reflected back from the filter. With the wavelength changed or the filter detuned (for example, by tilting the back mirror), the filter becomes almost totally reflective. With the optical circulator in front of the filter, the reflected power may be redirected from the incident port. The most advanced TFPF with mirrors built into the fiber and PZT alignment actuators have only 0.8 dB loss. The disadvantage of these filters is a need for active feedback and a reference element for frequency stability.
A VOA is an opto-mechanical device capable of producing a desired reduction in the strength of a signal transmitted through an optical fiber. Ideally, the VOA should produce a continuously variable signal attenuation while introducing a normal or suitable insertion loss and exhibiting a desired optical return loss. If the VOA causes excessive reflectance back toward the transmitter, its purpose will be defeated.
Due to the limitations of related art, there is a need for an AOTF with reduced backward-propagating waves that are created in response to propagation of an acoustic wave along an optical fiber of the AOTF.
Accordingly, an object of the present invention is to provide a VOA with reduced backward-propagating acoustic waves.
These and other objects of the present invention are achieved in an acousto-optic filter that includes an optical fiber with a first region and a second region. A first acoustic wave generator is coupled to optical fiber. The first acoustic wave generator produces a first acoustic wave that travels in a first direction in the first region. A backward-propagating wave is generated in response to propagation of the first acoustic wave along the optical fiber. The backward-propagating wave travels in an opposite direction relative to the first acoustic wave. A first acoustic wave propagation member is coupled to the optical fiber. A second acoustic wave generator is coupled to the optical fiber at the second region. The second acoustic wave generator produces a second acoustic wave that reduces a magnitude of the backward propagating acoustic wave.
In another embodiment of the present invention, an acousto-optic filter includes an optical fiber with a first region and a second region and a first acoustic wave generator coupled to the optical fiber. The first acoustic wave generator produces a first acoustic wave that travels in a first direction in the first action. In response to propagation of the first acoustic wave along the optical fiber a backward-propagating wave is generated. The backward-propagating wave travels in an opposite direction relative to the first acoustic wave. An acoustic damper is coupled to the optical fiber at the non-interaction region. The acoustic damper includes a proximal end with a taper configuration that reduces a power of the backward-propagating wave in a range of 20 to 30 dB or less.
In another embodiment of the present invention, an acousto-optic filter is provided that includes an optical fiber with a first region and a second region. A first acoustic wave generator is coupled to the optical fiber. The first acoustic wave generator produces a first acoustic wave that travels in a first direction in the first action. A backward-propagating wave is created in response to propagation of the first acoustic wave along the optical fiber. The backward-propagating wave travels in an opposite direction relative to the first acoustic wave. A first acoustic wave propagation member is coupled to the optical fiber. An acoustic damper coupled to the non-interaction region of the optical fiber. A second acoustic wave generator is coupled to the acoustic damper. The second acoustic wave generator produces a second acoustic wave that reduces a magnitude of the backward propagating acoustic wave.